1. Field of the Invention
This invention relates generally to bipolar separator plates for use in connection with substantially planar electrochemical fuel cell units for generating electricity, and fuel cell stacks comprising such fuel cell units. More particularly, this invention relates to gas (e.g. air) and/or liquid cooled, bipolar sheet metal separator plates for use in polymer electrolyte membrane fuel cells. Although the concept of this invention may be applied to bipolar separator plates for a variety of fuel cell types and designs, including molten carbonate and solid oxide fuel cells, it is particularly suitable for use in polymer electrolyte membrane fuel cell stacks in which the fuel and oxidant are provided to each of the fuel cell units comprising the fuel cell stack through external manifolds.
2. Description of Related Art
Fuel cells are electrochemical devices that convert chemical energy of a reaction directly into electrical energy. The basic fuel cell unit comprises an anode electrode, a cathode electrode and an electrolyte disposed between the two electrodes, to which a fuel, such as hydrogen is continuously provided to the anode (negative) electrode and oxidant, such as an oxygen-containing gas, is provided to the cathode (positive) electrode, whereby electrochemical reactions occur at the electrodes to produce an electrical current.
There are a number of fuel cell systems currently in existence and/or under development which have been designed and are proposed for use in a variety of applications including power generation, automobiles, and other applications where environmental pollution is to be avoided. These include molten carbonate fuel cells, solid oxide fuel cells, phosphoric acid fuel cells, and polymer electrolyte membrane fuel cells. One issue associated with successful operation of each of these fuel cell types is the control of fuel cell temperature and the removal of products generated by the electrochemical reactions from within the fuel cell.
Polymer electrolyte membrane fuel cells, which are well known in the art, are particularly advantageous because they are capable of providing potentially high energy output while possessing both low weight and low volume. Each such fuel cell typically comprises a “membrane-electrode-assembly” comprising a thin, proton-conductive, polymer membrane-electrolyte having an anode electrode film formed on one face thereof and a cathode electrode film formed on the opposite face thereof, although separate electrode and electrolyte layers may be employed in place of the membrane-electrode-assembly. In general, such membrane-electrolytes are made from ion exchange resins, and typically comprise a perflourinated sulfonic acid polymer such as NAFION™ available from E. I. DuPont DeNemours & Co. The anode and cathode films typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton-conductive material intermingled with the catalytic and carbon particles, or catalytic particles dispersed throughout a polytetrafluoroethylene (PTFE) binder.
Commercially viable fuel cell stacks may contain several hundred individual fuel cells (or fuel cell units), each having a planar area up to several square feet, depending upon the type of fuel cell. Fuel cell stacks containing a plurality of fuel cell units may be up to several feet tall, presenting serious problems with respect to maintaining cell integrity during heat-up and operation of the fuel cell stack. Due to thermal gradients between the cell assembly and cell operating conditions, differential thermal expansions, and the necessary strength of materials required for the various components, close tolerances and very difficult engineering problems are presented. In this regard, cell temperature control is highly significant and, if it is not accomplished with a minimum temperature gradient, uniform current density will not be maintainable, and degradation of the cell will occur.
In a fuel cell stack, a plurality of fuel cell units are stacked together in electrical series, separated between the anode electrode of one fuel cell unit and the cathode electrode of an adjacent fuel cell unit by an impermeable, electrically conductive, bipolar separator plate which provides reactant gas distribution on both external faces thereof, which conducts electrical current between the anode of one cell and the cathode of the adjacent cell in the stack, and which, in most cases, includes internal passages therein which are defined by internal heat exchange faces and through which coolant flows to remove heat from the stack. Such a bipolar separator plate is taught, for example, by U.S. Pat. No. 5,776,624. In such fuel cell stacks, the fuel is introduced between one face of the separator plate and the anode side of the electrolyte and oxidant is introduced between the other face of the separator plate and the cathode side of a second electrolyte. The fuel and oxidant can be fed to the fuel cell stack by means of internal gas manifolding, external gas manifolding, or a combination thereof.
Conventional bipolar separator plates for use in fuel cell stacks are typically molded or machined from graphite/carbon composites, electrochemically etched from plates or press formed from thin sheets of metal. The materials and fabrication processes employed depend upon the type of fuel cell. Carbon composites and metals can be used for fuel cells operating below about 200° C. while only metals are typically suitable for use above about 200° C. However, press forming of sheet metal requires costly die molds and requires that the metal have an elongation of greater than about 50%, which severely limits the metals available for use in the corrosive environments of polymer electrolyte membrane fuel cells or the high temperature environments of molten carbonate fuel cells and solid oxide fuel cells. In addition, press forming of thin sheets of metal to produce bipolar separator plates disadvantageously results in plates having stresses which must be relieved, typically by high temperature annealing, prior to use. Furthermore, depending upon the plate design, press forming results in a plate in which some portions of the formed metal are thinner than other portions due to metal thinning. This is particularly true with respect to plates comprising corrugated regions where the metal forming the peaks/valleys of the corrugations is thinner than the metal disposed between the peaks and valleys. As a result, for a given metal sheet thickness, the heights of the peaks as well as the distances between the peaks, hereinafter sometimes referred to as “pitch,” are significantly limited. Typically, the depth of the valley is generally limited to less than about ½ the peak-to-peak distance.
As previously suggested, it is essential that the bipolar separator plates provide good electrical contact with the electrodes which sandwich the electrolyte. It is also essential that the feed gases (fuel and oxidant) pass over their respective electrodes in a uniform manner with as low a pressure drop as possible between the inlet and the outlet. These two requirements mean that frequent bipolar separator plate contact with the electrodes is necessary to minimize contact resistance. At the same time, this contact should mask off the minimum amount of electrode so that easy gas access to the electrodes is maintained for maximum electrochemical performance.